Introduction to Heat Pipes · PDF file · 2015-09-26Introduction to Heat Pipes...

Introduction to Heat Pipes

Jentung KuNASA/ Goddard Space Flight Center

GSFC· 2015

https://ntrs.nasa.gov/search.jsp?R=20150018080 2018-05-17T09:03:43+00:00Z

Outline

• Heat Pipe Operating Principles– Pressure Drops– Operating Temperature

• Functional Types of Heat Pipes• Heat Pipe Operating Characteristics• Heat Pipe Design and Selection

– Design Considerations (mostly for Vendors)– Selecting Heat Pipes as Part of Thermal Control System and

M d li f H t Pi (f Th l A l t )Modeling of Heat Pipes (for Thermal Analysts)• Some Practical Considerations• Some Examples of Flight ApplicationsSome Examples of Flight Applications• Other Types of Heat Pipes

2Introduction to Heat Pipes - Ku 2015 TFAWS

Introduction

• Heat pipe is a capillary two-phase heat transfer device.– Transports heat from a heat source to a heat sink– Works as an isothermalizer

• Why two-phase thermal system?– Efficient heat transfer – boiling and condensationEfficient heat transfer boiling and condensation– Small temperature difference between the heat source and

heat sink

• Why capillary two-phase system? – Passive – no external pumping power

S lf l ti fl t l d i– Self regulating – no flow control devices– No moving parts – vibration free


Heat Pipes - Hardware

M t l ( l i ) t b ith th i f ld t i• Metal (aluminum) tube with grooves on the inner surface – cold extrusion• Grooves are filled with the working fluid (water, ammonia, propylene, etc.)• Flanges can be added on the outer surface for easy integration with instruments

or radiators (The flange is an integral part of the extrusion)

4

or radiators (The flange is an integral part of the extrusion)• Various diameters, lengths, and groove sizes

Introduction to Heat Pipes - Ku 2015 TFAWS

Major Functions of Heat Pipes

• Heat transfer

• Isothermalization

Temperature control• Temperature control

• Heat flux transformation

• Thermal diode and switches

Introduction to Heat Pipes - Ku 2015 TFAWS5

Heat Pipes – Operating PrinciplesHeat OutputHeat Input Wick

Vapor Flow

Typical use of heat pipe: one end (the evaporator) is attached to the heat source and theEvaporator Condenser

Liquid Return

• Typical use of heat pipe: one end (the evaporator) is attached to the heat source, and the opposite end (the condenser) to the heat sink. The middle section (the adiabatic section) is insulated.

• As liquid is vaporized at the evaporator, the vapor pressure builds up, forcing vapor to flow axially along the center core to the condenser .y g

• Vapor condenses at the condenser. Liquid is drawn back to the evaporator by the capillary force along the grooves.

• The pressure difference between the vapor and liquid phases is sustained by the surface tension force of the fluid.


• Passive – no external pumping power is required; the waste heat provides the driving force for the fluid flow.

Differential Pressure Across a Curved Surface

R1R1

P = P1 - P2 = (1/R1 + 1/R2)

: Surface tension; R1 and R2: Radii of curvature

R2 R2

: Surface tension; R1 and R2: Radii of curvature

P1

P2


Pressure Differential Across a Meniscus

• A meniscus will be formed at the liquid/vapor interface, and a capillary pressure is developed.

Pcap = 2 cos/R

: Surface tension; R: Radius of curvature; : Contact Angle

RR

• The maximum capillary pressure

RRPP

Pcap,max = 2 cos/Rp

PPR Rp

Rp : Radius of the pore


Pressure Balance in Heat Pipes

• The fluid flow will induce a frictional pressure drop. The total pressure drop over the length of the heat pipe is the sum of individual pressure drops.

P P + P + P

• The meniscus will curve naturally so that the capillary pressure is equal to the total pressure drop

Ptot = Pvap + Pliq + Pg

RR

pressure is equal to the total pressure drop.

Pcap = Ptot

Pcap = 2 cos/R (R Rp)

• The flow will stop when the capillary limit is exceeded.RRPP

P = 2 cos/R

cap ( p)

For normal operation of heat pipes

Pcap,max = 2 cos/Rp

Rp : Radius of the pore


• For normal operation of heat pipes:Ptot = Pcap ≤ Pcap,max

Pressure Differential at Liquid Vapor Interface

Liquid Flow

Heat SinkHeat Source

wall

Le La Lc

Vapor Flow

wallLiquid Flow

EvaporatorSection

Condenser Section

Adiabatic Section

• The vapor pressure decreases as it flows from the evaporator to the condenser.• The liquid pressure decreases as it flows from the condenser to the evaporator.• At any cross section of the heat pipe, a pressure differential exists between the

vapor and liquid phases. This delta pressure is sustained by the surface tension force developed at the liquid/vapor interface at the tip of each groove.

• The lowest delta pressure occurs at the very end of the condenser (zero). The


highest delta pressure occurs at the very end of the evaporator.

Heat Pipes - Heat Transport Limit

Le La Lc• For proper heat pipe operation, the

total pressure drop must not exceed its capillary pressure head .

Ptot ≤ Pcap,max

P P P P

Liquid Flow

Heat SinkHeat Sourcewall

a

Vapor Flow

c

Ptot = Pvap+ Pliq + Pg

Pcap,max = cos/Rp

• Heat Transport Limit– (QL)max = QmaxLeff

wallLiquid Flow

EvaporatorSection

Condenser Section

Adiabatic Section

p

VaporVapor

( )max max eff

– Leff = 0.5 Le + La + 0.5 Lc

– (QL)max measured in watt-inches or watt-meters

• Capillary pressure head:

Section Section

LiquidPressure DropPr

essu

re CapillaryPressure

VaporPressure Drop

Liquid

Li id

Capillary pressure head:

Pcap 1/ Rp

• Liquid pressure drop:

Pliq 1/ Rp2

Le La Lc

LiquidNo Gravity Force

Adverse Gravity Force

Pliq 1/ Rp

• An optimal pore radius exists for maximum heat transport.

• Limited pumping head against gravity


Distance

b) Vapor and liquid pressure distributions

Some Wicks Used in Heat Pipes

• Many HP hardware variations exist.– Size– LengthLength– Shape– Wick material– Wick construction

POWDER METAL WITHPEDESTAL ARTERY

– Working fluid

• Axial Grooves

CIRCUMFERENTIALSCREEN WICK

– Versatility– Design simplicity– Reliability

High heat transport

AXIAL GROOVES

– High heat transport– High thermal

conductance– Broadly used in


SLAB WICKAXIAL GROOVES

aerospace applications

Functional Types Of Heat Pipes

• Three Basic Functional Types– Constant Conductance Heat Pipe (CCHP)– Variable Conductance Heat Pipe (VCHP)

Di d H t Pi– Diode Heat Pipe

Heat OutputHeat Input Wick

Vapor Flow

Evaporator Condenser

Liquid Return


Energy Balance in Heat Pipe

L

Wick Structure

Container Wall QOUTQIN

Le LaLc

Wick Structure

Liquid FlowVapor Flow

CondensationEvaporization

QIN = QOUT = m . TS

QINQOUT

OUT

Le = Evaporator lengthLa = Adiabatic lengthLc = Condenser length

M fl t (li id ).


m = Mass flow rate (liquid or vapor) = Latent heat of vaporization

.

Thermal Characteristics of a CCHP

Wick Structure

Container Wall QOUTQIN

Le La Lc

Vapor FlowCondensationEvaporization

Liquid FlowVapor Flow

TS

QINQOUT

Tva

Q2 >Q1’ TS1

erat

ure

Q = h(DLc)(TV -Ts)

QIN

Tv1

Tv2

Q1, TS1

Q1, TS2 < TS1

Vapo

r Tem

pe Lc = constant

h(DLc) = constant conductance

T varies with T and Q

15

DistanceTV varies with Ts and Q


Temperature Gradient in a CCHP

Vapor CoreHeat Flow Path

• The thermal conductance is very high for the fluid flow

Wick

Container

flow.

• The temperature diff f th h t Container

Condenser(Heat Sink)

Evaporator(Heat Source)

AdiabaticSection

difference from the heat source to the heat sink is dominated by the much smaller thermal

rce

Tem

p. Heat FlowQ

nk T

emp.

conductance at the heat source/evaporator interface and the condenser/heat sink

Sour

EvaporatorSurface

CondenserSurface

Sincondenser/heat sink

interface.

Surface

Liquid VaporInterface

Surface


Temperature Gradient in a CCHP

Vapor CoreHeat Flow Path

Wick

Container

Condenser(Heat Sink)

Evaporator(Heat Source)

AdiabaticSection

TV3TV3

urce

Tem

p.(F

ixed

)

Heat Flow Q1

ourc

e Te

mp.

Heat FlowQ2 >Q1

Sink

Tem

p.TV1

TV2 Q1

Q1

TS1

Q1

ink

Tem

p.

TS3 > TS1

V3

TV1

Sou

EvaporatorSurface

Li id V

CondenserSurface

S

EvaporatorSurface

Li id V

CondenserSurface

S1

TS2 < TS1

Q2 < Q1 SiTS1

TS2 < TS1




Summary of CCHP Operation (1)

• First law of thermodynamics

Second law of thermodynamics• Second law of thermodynamics

• Capillary pressure capabilityp y p p y

• Pressure balance

• Saturation states


Summary of CCHP Operation (2)

• Heat transfer in condenser zone

• Heat transfer in evaporator zone

• Relationship between temperature differential and pressure differentialdifferential


Thermal Characteristics of a VCHP

Gas Reservoir

Effective Condenser

Evaporator

Active Portion of Heat Pipe

Adiabatic Section Condenser

Q = h(DLc)(TV -Ts)Lc varies with Ts and QNon-Condensable

Effective Condenser

Vapor Flow c s

so as to keep TV constanth(DLc) = variable conductance

Reservoir size is a function of:H t I t H t O t tGas Front

gasVapor Flow

Reservoir size is a function of:• Range of heat load• Range of sink temperature• Temperature control requirement

Temperature

TTv1

Positions of gas front

Heat Input Heat Output

Distance

Heat-sinktemperature

Tv2

Tv3Q3 Q2

Q1

20

DistanceEvaporator Adiabatic

sectionCondenser Gas

reservoir


VCHPs

T i l VCHPTypical VCHP

• Types of VCHPs Feedback-controlled VCHP Feedback-controlled VCHP Passive VCHP

OCO-2 VCHPs


OCO 2 VCHPs

Electrical Feedback-controlled VCHP

• Typically maintain evaporator temperature control of ± 1-2 °C over widely i t d h t i k t tvarying evaporator powers and heat sink temperatures

• Roughly 1-2 W electrical power required for the reservoir heaters


Passive VCHP - Gas-Charged Heat Pipe (GCHP)

Liquid in wick

Vapor

Heat from Instrument Heat to RadiatorLiquid in wick

Vapor

Heat from Instrument Heat to Radiator

ICENear Vacuum

Empty Wick

ICENear Vacuum

Normal Operation of a CCHPAdiabatic CondenserEvaporator Adiabatic CondenserEvaporator Adiabatic CondenserEvaporator

Formation of an Ice Plug in a CCHP

• Issues: formation of ice plug in the condenser and difficulty of re-start

Ice in WickIce in WickLiquid in WickHeat to RadiatorHeat from Instrument

Liquid in WickHeat to RadiatorHeat from Instrument

NCGNCGNCGVapor NCGVapor

Adiabatic CondenserEvaporator GasRegion

Adiabatic CondenserEvaporator GasRegionNormal Operation of a GCHP

Adiabatic CondenserEvaporator GasRegionCondenserEvaporator GasRegion

Formation of Ice in a GCHP

• NCG in GCHP: allows the heat pipe to freeze in a controlled fashion; no


• NCG in GCHP: allows the heat pipe to freeze in a controlled fashion; no ice plug – no risk of pipe burst; helps re-start of the heat pipe.

Diode Heat Pipes

• Diode heat pipes are designed to act like an electronic diode.

• Evaporator hotter than condenser – Heat flows from the evaporator to the condenser

• Condenser hotter than evaporator – No heat flows from the condenser to the evaporator


Diode Heat Pipe – Excessive Liquid at Condenser End

Q Q Excess Liquid

ReservoirEvaporator Condenser

• During normal operation, the diode heat pipe works as regular CCHP with excess liquid stored in the reservoir attached to the condenserexcess liquid stored in the reservoir attached to the condenser

– Excess liquid may block part of the condenser depending on the thermal load and reservoir sink temperature.

• During reverse operation vapor flows in the opposite direction. Vapor g p p pp pcondenses in the evaporator, eventually fills the entire evaporator section.

– No heat can be dissipated to the evaporator.


Gas Diode Heat Pipes – NCG at Condenser End

Q Q NCG

ReservoirEvaporator Condenser

• During normal operation, the gas diode heat pipe works similarly to a VCHP.

– Gas reservoir at condenser end with NCG – NCG may blocks part of the condenser depending on the thermal load

and reservoir sink temperature.D i ti fl i th it di ti• During reverse operation vapor flows in the opposite direction

– NCG moves to the opposite end of the heat pipe due to the change in pressure.

– NCG blocks off what would be the condensing end, effectively shutting g , y gdown the heat pipe.


Liquid Trap Diode Heat Pipes

QQReservoir Wick

Evaporator CondenserReservoir

• Reservoir at evaporator end of heat pipe contains wick. – Reservoir wick does not communicate with heat pipe wick.

• During normal operation the pipe works as a CCHP. – Liquid evaporates at hot end and condenses at cold end.– Liquid returns to hot end via heat pipe wick.

• During reverse direction, liquid evaporates at the hot end and condenses in the reservoir and becomes trapped.

– Liquid cannot return to the hot end.– The pipe is shut down.The pipe is shut down.– No heat dissipation to the regular evaporator.


Heat Pipe Operation

• The heat pipe is an isothermalizer• The heat pipe is an isothermalizer.– A single heat pipe can serve multiple heat sources and/or

multiple heat sinks.– The vapor temperature is nearly isothermalThe vapor temperature is nearly isothermal.

• The heat pipe can be bent.– Small degradation in heat transport limit

• Although the heat pipe can transport hundreds of watts over many feet of distance, it has a very limited capability to sustain the total pressure drop.

Example: no more than 0 5” adverse elevation using– Example: no more than 0.5” adverse elevation using ammonia as the working fluid (< 100 Pa) in one-G environment.

– Ground testing of a heat pipe requires that the heat pipe be g p p q p pplaced horizontally with < 0.2” adverse elevation.

• The heat pipe works well with favorable elevations, e.g. in a vertical position with the evaporator below the condenser.


p p

Liquid Transport Factor vs Temperature1 0E+121.0E+12

AcetoneAmmoniaBenzeneDowtherm-EDowtherm-AMethanolToluene

Water

• A convenient figure of merit is the liquid transport factor,

1.0E+11

R, N

l (W

M-2

)

TolueneFreon-11 Water

Ammonia

q p ,Nl,

Nl = l/l

SP

OR

T FA

CTO

RAcetone

Dowtherm-E

Methanol

Nl = Latent Heat * Surface Tension * Density/ Viscosity

1.0E+10

LIQ

UID

TR

AN

Freon-11

ceto e

Dowtherm-A

Toluene

Benzene


1.0E+09100 200 300 400 500 600 700 800

TEMPERATURE (K)

Heat Pipe Operation Near the Critical State

• Never operate a heat pipe near the critical state of the working fluid.- Diminishing liquid transport factor


A PvT surface for a substance which contracts on freezing

30

Heat Pipe Operation Near the Freeze Point• Move the HP operation away from the freezing point of the working fluid.p y g p g

- Low vapor pressure- Non-isothermal

Saturation Curve

P2

P1

Pres

sure

Vapor

Liquid

P3

P

T2 T1TT4

P4


T2 T1Temperature

T3T4

Heat Pipe Operating Limits

• Capillary Limit– Most common

• Vapor Pressure Limit– Operation near the frozen state– Rule of thumb: (Pv/ Pv) < 0.1( v v)

• Entrainment Limit– High vapor velocityHigh vapor velocity

• Boiling LimitHigh heat fl– High heat flux

• Sonic Limit


– Liquid metal heat pipes

Capillary Limit - Heat Pipe Dry-out Condition• The temperature difference between the end of the evaporator and the p p

adiabatic section is usually plotted.• Recovery from dry-out condition can be achieved by reducing the heat

load.

apor

(OC

)Liquid Flow

Heat SinkHeat Sourcewall

Le La

Vapor Flow

Lc

p, E

vap.

–Va

wallLiquid Flow

EvaporatorSection

Condenser Section

Adiabatic Section

pera

ture

Dro

p

pe

Tem

p

Hea

t Pip

Dry

Out


Heat Load (Watts)

Heat Pipe Design Procedure

• Determine the operating temperature range• Determine the operating temperature range.• Select the working fluid

– Liquid transport factorNever operate near the freezing temperature or the critical– Never operate near the freezing temperature or the critical temperature of the working fluid.

• Select the container material.– Material compatibilityMaterial compatibility– Structural strength

• Select the wick.– Material– Shape

• From the thermal requirement, determine the type of heat pipe.– CCHP, VCHP, Diode HP, ,

• From the heat transport requirement, determine the heat pipe diameter and length, and number of heat pipes.

– Temperature drop across the heat pipe


– Temperature gradient requirement– Some computer models available

Heat Pipe Design Considerations (1)

• Heat pipe theory• Physical, thermal, and mechanical constraints

Material properties• Material properties• Application requirements• Fabrication, processing, and testing limitations, p g, g• Reliability and safety


Heat Pipe Design Considerations (2)

• Once the performance requirements and specifications are defined, the design and evaluation process can be initiated.

• Three Basic Consideration– Working fluid– Wick design

C t i ( l )– Container (envelope)

• Several options may exist.

• The final design usually represents an iteration among various design factors.


Heat Pipe Design Procedure

Problem Specifications

Design Criteria

ContainerFluidDesignTheory

Procedure

Container Properties

Fluid Properties

Wi k

Optional Solutions

Other Considerations

Wick Properties

Solutions

Evaluation Procedure

EvaluationCriteria

Optimum Solution


Procedure Solution

Problem Definition and Design Criteria (1)

Requirement Impact on Design

Operating temperature range Choice of working fluid;Pressure retentionPressure retention

Thermal load Heat pipe diameter; No. of heat pipes; Wick design; Choice of working fluidworking fluid

Transport length Wick design

Temperature uniformity and Wick design; Conductive pathTemperature uniformity and overall T

Wick design; Conductive path length trade-off; Heat pipe geometry

Physical requirements Size Weight Structural strengthPhysical requirements Size, Weight, Structural strength and geometry

Acceptance and qualification testing

“one-G’ operation and “zero-G” correlation


testing correlation

Problem Definition and Design Criteria (2)

Requirement Impact on DesignRequirement Impact on Design

Ground testing Orientation

Dynamic environment Operation under accelerating field;Dynamic environment Operation under accelerating field; Structural integrity

Thermal environment Pressure retention under non-operating temperaturestemperatures

Mechanical interfacing Mounting provisions; Provision for thermal interfacing

Man Rating Pressure vessel code; Fluid toxicity

Transient behavior Choice of working fluid; Wick design; Variable conductance typeVariable conductance type

Reliability Leak tightness; Material compatibility; Processing control; Redundancy


Working Fluid

• Variety of fluid possible - selection determined by applications: operating temperature, capacity, safety, etc.

• Must be able to exist as both vapor and liquid at the operating t ttemperature.

– Often best to select a fluid that has its normal boiling point near desired operating temperature.

• Use the liquid transport factor as the figure of merit.• Purity of the working fluid is critical (99.999%).

– Impurities reduce performance and may lead to undesirable p p yNCG buildup.

• Must be compatible with other materials in the heat pipe.• Operating pressure• Operating pressure• Wicking capability in body-force field• Liquid thermal conductivity


• Vapor phase properties

Operating Temperature Ranges

• Cryogenic– 0.1K to 150K– Elemental or simple organic compounds

• Low Temperature– 150K to 750K150K to 750K– Polar molecules or halocarbons

• High Temperature• High Temperature– 750K to 3000K– Liquid metals


Wick Material

• Provides capillary pumping head.• Provides porous media for liquid

transport.p• Variety of design possibilities

exist.– Axial groove (most common)a g oo e ( ost co o )– Screen– Sintered powder– Arteries

POWDER METAL WITHPEDESTAL ARTERY

– Composites• Small uniform pore size is

desirable. CIRCUMFERENTIALdesirable.– Compromise with desire for

high permeability

SCREEN WICK


SLAB WICKAXIAL GROOVES

Envelope Material

• Typically a metal tube tightly sealed at both ends • A variety of shapes, sizes, and configurations exist.

Basic design considerations• Basic design considerations– Structure integrity and leak tight containment– Compatibility with working fluid and external environment

Internal size and geometry for liquid and vapor flow– Internal size and geometry for liquid and vapor flow requirements

– External interface with heat sources and sinks– Fabrication concerns– Fabrication concerns– Heat transfer concerns


Fluid Inventory

• Liquid charge must be sufficient to saturate the wick.– Performance degrades with improper charge.– Undercharge: reduced heat transport capability

O h li id ddl i th d– Overcharge: liquid puddle in the condenser• The optimal inventory will be determined based on operating conditions.


Fabrication/Testing Issues

• Cleaning and material compatibility are critical. Must develop and follow tight cleaning procedures.

• Proper level of fluid charge is important.p g p• Component and system level tests

– In-process – Proof pressure– Proof pressure– Burst– Leak– Performance - vary tilt to develop performance map– Performance - vary tilt to develop performance map

• Rigid requirements for space applications– MIL-STD1522A (USAF)

NSTS 1700 7B (NASA)– NSTS-1700.7B (NASA)


Heat Pipe Design Tools

• Each vendor has its own analytical design tools• Each vendor has its own analytical design tools.• Groove Analysis Program (GAP) software – good for axially grooved

heat pipes– NASA-owned– Available for purchase through COSMOS


Heat Pipe Selection – for Thermal Engineers

• First and foremost: determine the operating temperature range• First and foremost: determine the operating temperature range.• Select the working fluid. • Select the wick and container material.

M t i l tibilit– Material compatibility• Obtain performance curves for various heat pipes from vendors. • From the thermal requirements, determine the type of heat pipe.

– CCHP, VCHP, Diode HP• From the heat transport requirement, determine the heat pipe diameter

and length, and number of heat pipes.O ll t t d f h t t h t i k– Overall temperature drop from heat source to heat sink

– Temperature gradient requirement– Temperature uniformity requirement– Physical constraints – diameter and shape of heat pipes– Physical constraints – diameter and shape of heat pipes– Mass constraints– Design margins– Cost


• Ground test requirement at the instrument and spacecraft level

Heat Pipe Performance Curve for Given Heat Pipe Design and Working Fluid

30300

TRANSPORT CAPABILITY VS. TEMPERATURE DIE 16692, Single Sided Heat Pipe, Ammonia Fluid

236.6224.3

20

25

200

250

m)(W

-m)

0-g

15

20

150

200

Stat

ic H

eigh

t (m

m

spor

t Cap

abili

ty g

- 2.54 mm

5

10

50

100

S

Max

Tra

ns Static Height

00-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

Temperature (°C)Temperature ( C)


Other Practical Considerations

• 3-dimensional heat pipes

Dual bore heat pipes• Dual-bore heat pipes

• Ground testing of heat pipes in reflux modeg p p


3-Dimensional Heat Pipes

• 3-D heat pipes cannot be tested in one-G for its performance verification.

• For design qualifications an equivalent 2D HP can be made withFor design qualifications, an equivalent 2D HP can be made with same number of bends, same degree of bend for each bend, and same segment lengths, and test for its performance.

• For acceptance test, the 3-D pipes may be tested in segments.For acceptance test, the 3 D pipes may be tested in segments.– Adequate for axially-grooved heat pipes which have uniform

grooves.– Inadequate for slab wick heat pipes.q p p


Dual Bore Heat Pipes

• Some reasons to use dual bore heat pipes– for redundancy– to reduce heat flux and temperature gradient between the heatto reduce heat flux and temperature gradient between the heat

source and the heat pipe– HP can serve as structural member

• For qualification test each bore is charged and tested separatelyFor qualification test, each bore is charged and tested separately. • For acceptance test, both pipes are tested together – cannot tell

whether one of them fails.F h i b i h d fi t th th th• For charging, one bore is charged first, then the other.

• Performance such as the heat transport, heat flux, thermal conductance, liquid slug and NCG can only be done for both heat pipes on the “a erage” basispipes on the “average” basis.

• Dual bore heat pipes are more difficult to bend.


Ground Testing of Heat Pipes in Reflux Mode

• It may be necessary to test the heat pipe in a reflux mode during instrument and/or spacecraft level test.

• Liquid puddle will form at the evaporator which is below the condenser.

• Liquid may not boil to generate vapor unless a superheat is exceeded atLiquid may not boil to generate vapor unless a superheat is exceeded at the evaporator.

• To facilitate the ground testing some concentrated heater can be attachedTo facilitate the ground testing, some concentrated heater can be attached to the evaporator to create a high heat flux, which initiates liquid boiling.


Detailed Thermal Resistance Model of Heat Pipe


Heat Pipe Modeling Using SINDA/FLUINT

• NOT an HP design tool – e.g. groove dimensions, VCHP reservoir sizing.• Appropriate for most TCS design and analysis• Very important: read the manual for capabilities/limitations.


Heat Pipe Modeling Using SINDA/FLUINT

• Uses subroutines HEATPIPE and HEATPIPE2.• Simulates CCHP gas-charged CCHP gas-charged VCHP• Simulates CCHP, gas-charged CCHP, gas-charged VCHP.• The vapor is assumed to be a uniform temperature (i.e. single node). • The vapor node must be an arithmetic node.• Make certain all units are consistentMake certain all units are consistent.• Called from Variables 1 block


VCHP Modeling – Node/Conductor Network

Evaporator Condenser Adiabatic

Heat Pipe Wall Nodes

Conductors

Vapor Node

• Modeling a VCHP in SINDA begins as a basic node-conductor networkModeling a VCHP in SINDA begins as a basic node conductor network

• Conductors are initialized for each heat pipe wall node to the vapor node and can be adjusted depending on the gas front location

• To simulate NCG located in a particular node in the condenser the conductor for that wall node to the vapor node is set to zero or a percentage of the original value if partially blocked


Flight Heat Pipes• Ammonia HPs

– Most prevalent– Too many to list

• Water HPs– NRL WindSat (launched 2003)

• Butane HPs– MESSENGER- Diode HP (2004 - 2011)

• Ethane HPs– LDEF (1984-1990)– Swift XRT (launched 2004)– LDCM TIRS (launched 2013)

• Oxygen HPs (flight experiment)– CCHP on STS-62 (1994)– Flexible diode HP on CRYOHD experiment on STS-94 (1997)

• Nitrogen HPs (flight experiment)– CCHP on STS-62 (1994)

• Methane HPs (flight experiment)– Flexible diode HP on CRYOHD experiment on STS-94 (1997)


Triple Point and Critical Temperature of Some fluids

Fluid Freezing Critical TemperatureFluid Freezing Temperature (K)

Critical Temperature (K)

Ammonia 195.4 405.5Butane 134.6 425.1Ethane 89.9 305.3Helium 2.2 5.2Hydrogen 13.8 33.2Methanol 175.6 512.6Methane 90.7 190.8Methane 90.7 190.8Neon 24.6 44.4Nitrogen 63.2 126.2Oxygen 54 4 154 6Oxygen 54.4 154.6Pentane 143.5 469.8Propylene 88 365.6


Water 273.1 647

Swift XRT Ethane Heat Pipes


Orbiting Carbon Observatory – 2 (OCO-2) VCHP


CCHPs/VCHPs/LHPs on SWIFT ABT• Burst Alert Telescope, a gamma ray detector array, is one of three instruments on Swift p , g y y,• Launched: 20 November, 2004 • Detector array has 8 CCHPs for isothermalization and transfer of 253 W to dual, redundant, LHPs

located on each side

ShieldLiquidLine 2

LHP 2 Condenser

LiquidLine 1

DetectorArrayLHP 1 Condenser Vapor

Line 2

LHP evaporator

LHP 1 Evaporator

CompensationChamber 2

Vapor Line 1

CompensationChamberRadiator

CompensationChamber 1

LHP 2 Evaporator

Liquid Line 1 Liquid Line 2Vapor Line 2

Vapor Line 1

RadiatorFor both loops

Liquid Line 2


Swift BAT System – VCHP and LHP Evaporator

Titanium bracket support of hydro-accumulator

G-10 washers for thermal isolation

pp y

Saddle soldered to VCHP

Aluminum clamps for VCHP

CCHPs attached to evaporatorsaddle with Eccobond G-10 washers for

th l i l tiSaddle attached to evaporatorpump with Eccobond

thermal isolation

Heat exchanger swaged over VCHP condenser

Titanium support bracket for VCHP reservoir


Swift BAT VCHPs and LHPs


HPs/LHPs on ICESat GLAS

• GLAS has high powered lasers to measure polar ice thickness

• First known application of a two-phase loop to a laser

• 2 LHPs; Laser altimeter and power electronics – Propylene LHPs

• Launched January, 2003• Both LHPs successfully turned on• Very tight temperature control ~ 0.2 oC

Radiator

High Power Lasers

Loop HeatPipe

Capillary Two-Phase Systems Introduction to Heat Pipes - Ku 2015 TFAWS

GOES-R ABI HPs/LHPs Assembly

• Radiator LHP Assembly contains two parallel redundant LHPs a shared radiator• Radiator LHP Assembly contains two parallel redundant LHPs, a shared radiator, heaters, thermostats, thermistors, and an electrical harness assembly.

• Evaporator Assemblies mount to Heat Pipe Network on Optical Bench

+Z Flexures

Radiator Panel

-Z Flexures

Radiator Panel

LHP Evaporator

Optical BenchAssembly

Heat PipeNetwork

Assemblies

+X

+Z

+Y+X +Y


HST ACS CPLs and ASCS Radiator Design

STRAIN RELIEF BRACKET WITH SURVIVAL HEATERS

ACS-1

ACS-2

ACS-1

ACS 2

STISCOS

STIS

COS

ACS-2

ACS INTERFACE PLATE

VAPOR LINES RELOCATED TO EDGE OF PANEL TO PREVENT LOSS OF SUCOOLER EFFICIENCY, REQUIRED ADDITIONAL VAPOR LINE HEATERS ON PANEL EDGE

LIQUID LINE SUBCOOLER AREA

EXTERNAL FLEX HOSES WITH FLEXIBLE SURVIVAL HEATERS

CONDUITACS INTERFACE PLATERIGID LIQUID AND VAPOR LINE TUBING

CRYOVENT LIGHT SEAL

EVAPORATOR PUMP 1

LIGHT SEAL

EVAPORATOR PUMP 2

INTERNAL FLEX HOSE BUNDLE


HST CPL/HP Radiator Assembly

Subcooler S tiSection

Isothermalizer h t iheat pipes

Heat Pipe Heat E hExchangers

Reservoir Lines


Other Types of Heat Pipes

• Vapor Chamber• Pressure Controlled VCHP• Two-Phase Closed ThermosyphonTwo Phase Closed Thermosyphon• Rotating Heat Pipe• Oscillating Heat Pipe


Vapor Chamber

• Vapor chambers are planar heat pipes for heat spreading and/or isothermalizing


Pressure Controlled VCHP

V i l t f• Vary reservoir volume or amount of gas – Actuator drives bellows to modulate the reservoir volume – Pump/vacuum pump adds/removes gas

Used for precise temperature control• Used for precise temperature control

** Anderson, W.G, et al., ”Pressure Controlled Heat Pipe Applications,” 16th International Heat Pipe conference, Lyon, France, May 20-24, 2014


Two-Phase Closed Thermosyphons

• A gravity-assisted wickless heat pipe

• The condenser section is located above the evaporator so that the condensate is returnedcondensate is returned by gravity.

• The entrainment limit is more profoundmore profound.

• The operation is sensitive to the working fluid fillworking fluid fill volume.


Rotating Heat Pipe

• A rotating heat pipe uses centrifugal forces to move the condensate from the condenser to the evaporator

• The inside of the heat pipe is a conical frustum, with the evaporator inside di t (I D ) l th th d I Ddiameter (I.D.) larger than the condenser I.D.

• A portion of the centrifugal force is directed along the heat pipe wall, due to the slight taper (R2sin).


Oscillating Heat Pipes

• A capillary tube (with no wickA capillary tube (with no wick structure) bent into many turns and partially filled with a working fluid

• When the temperature difference between evaporator and pcondenser exceeds a certain threshold, the gas bubbles and liquid plugs begin to oscillate spontaneously back and forthspontaneously back and forth.


Questions?Questions?

Date post:	15-Mar-2018
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